1. Technical Field
The present invention relates in general to electrical circuitry and, in particular, to an electrical circuit for detecting an impedance at a circuit input.
2. Description of the Related Art
Currently, complementary metal-oxide semiconductor (CMOS) technology is nearly universally used to fabricate memories and other electronic components that require high switching speed and low power consumption. As integrated circuit fabrication techniques have continued to improve, it has been possible to produce integrated circuits having ever smaller feature sizes and increasing performance. However, recently there has been a growing realization that CMOS technology has fundamental scaling limitations and that diminishing performance enhancements are gained by standard geometric scaling. Accordingly, there is currently great interest in operating CMOS systems at sub-ambient temperatures to extract greater performance from current design points.
When operating a CMOS system at sub-ambient temperature, practical system reliability concerns dictate that no moisture be permitted to condense on the surfaces of system boards and low temperature cooling assemblies. Thus, it is imperative to maintain such surfaces at a temperature greater than the dew point of water vapor. In conventional systems intended for operation at sub-ambient temperatures, optical hygrometers such as that depicted in FIG. 1 are utilized for dew point detection.
As shown in FIG. 1, a conventional optical hygrometer 10 includes two optical channels, a control channel 12 and a detection channel 14, that each contain a light emitting diode (LED) 16 and a photodetector (PD) 18. In control channel 12, the amount of light emitted by LED 16 that is received by photodetector 18 is controlled by an optical balance 22, which determines how far opaque barrier 20 extends into control channel 12. Light emitted by LED 16 of detection channel 14, on the other hand, is received by photodetector 18 after being reflected by a mirror 24 located on a cooled surface for which temperature control is desired. An output of each of photodetectors 18 is connected to an input of differential amplifier 26, which generates an amplified output that forms an input to a defrost control circuit.
In operation, optical hygrometer 10 is first calibrated through the adjustment of optical balance 22 when mirror 24 is free from condensation such that the output of differential amplifier 26 is essentially zero. Then, as the surface on which mirror 24 is cooled below the dew point, moisture will begin to condense on mirror 24, scattering the light emitted by LED 16 in detection channel 14 and causing a differential voltage to develop between the inputs of differential amplifier 26. Thus, when condensation reaches a level determined by the setting of optical balance 22, differential amplifier 26 signals the defrost control signal that condensation has formed on the cooled surface and that remedial action, such as heating the cooled surface, is required.
While conventional optical hygrometers such as that illustrated in FIG. 1 are effective in providing an imprecise indication of the condensation of moisture on a cooled surface, such conventional optical hygrometers are subject to a number of drawbacks. First, conventional optical hygrometers tend to be expensive and bulky. It is therefore inconvenient and costly to utilize a large number of optical hygrometers in a practical computer system. In addition, conventional optical hygrometers cannot be used to detect moisture condensation over small surface areas. Second, conventional optical hygrometers do not generate digital output signals. Optical hygrometers therefore cannot easily be integrated into or interconnected with conventional digital circuitry, for example, to permit the gathering of statistical data pertaining to moisture condensation. Third, conventional optical hygrometers have poor sensitivity to the formation of condensation. As a result, damage can result to system components if the optical balance is calibrated incorrectly.
As should thus be apparent, it would be useful and desirable to provide an improved hygrometer for detecting moisture condensation in a computer system operating at sub-ambient temperature.
In accordance with the present invention, an impedance detection circuit is provided that includes a circuit input having a first contact and a second contact, a reference voltage rail coupled to the first contact, and a memory cell having a data node coupled to the second contact and an output. When the memory cell is read, the logic state of the output provides an indication of an impedance coupling the first and second contacts at the circuit input. The impedance detection circuit can be utilized to sense resistive and capacitive inputs and has any number of applications, including use as a digital hygrometer and as a fingerprint sensor.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.